Jayaram N. Chengalur

National Centre For Radio Astrophysics - Tata Institute Of Fundamental Research

We present the first estimate, based on direct H i 21 cm observations, of the H i mass function (H i MF) of star-forming galaxies at z ≈ 1, obtained by combining our measurement of the scaling relation between H i mass ( M H i ) and B -band luminosity ( M B ) of star-forming galaxies with a literature estimate of the B -band luminosity function at...

We present here estimates of the average rates of accretion of neutral gas onto main-sequence galaxies and the conversion of atomic gas to molecular gas in these galaxies at two key epochs in galaxy evolution: (i) z ≈ 1.3–1.0, toward the end of the epoch of peak star formation activity in the Universe, and (ii) z ≈ 1–0, when the star formation acti...

Star-forming galaxies are believed to replenish their atomic gas reservoir, which is consumed in star formation, through accretion of gas from their circumgalactic mediums (CGMs). However, there are few observational constraints today on the gas accretion rate in external galaxies. Here, we use our recent measurement of the scaling relation between...

The atomic hydrogen (H i ) properties of star-forming galaxies in the local universe are known to correlate with other galaxy properties via the “H i scaling relations.” The redshift evolution of these relations serves as an important constraint on models of galaxy evolution. However, until recently, there were no estimates of the H i scaling relat...

The atomic hydrogen (HI) properties of star-forming galaxies in the local Universe are known to correlate with other galaxy properties via the ``HI scaling relations''. The redshift evolution of these relations serves as an important constraint on models of galaxy evolution. However, until recently, there were no estimates of the HI scaling relatio...

We use the Giant Metrewave Radio Telescope Cold-H i AT z ≈ 1 (CAT z 1) survey, a 510 hr H i 21 cm emission survey of galaxies at z = 0.74–1.45, to report the first measurements of atomic hydrogen (H i ) scaling relations at z ≈ 1. We divide our sample of 11,419 blue star-forming galaxies at z ≈ 1 into three stellar-mass ( M * ) subsamples and obtai...

The Expanded Giant Metrewave Radio Telescope (eGMRT) [1] prototype aperture array beamformer is implemented on an FPGA and tested in the free-space test range at the GMRT site. The beamformer is a single FPGA implementation processing 32 antenna inputs having a bandwidth of 32 MHz producing five independent beams. In the calibration mode, we config...

The neutral atomic hydrogen (H i ) mass function (H i MF) describes the distribution of the H i content of galaxies at any epoch; its evolution provides an important probe of models of galaxy formation and evolution. Here, we report Giant Metrewave Radio Telescope H i 21 cm spectroscopy of blue star-forming galaxies at z ≈ 0.20–0.42 in the Extended...

The Expanded Giant Metrewave Radio Telescope (eGMRT) [1] proposal aims at increasing the Field-of-View (FoV) of the GMRT antennas by a factor of 25 using a Focal Plane Array (FPA) and beamforming system. As part of the initial proposal, an optimized FPGA-based prototype beamformer is designed and tested in the free-space test range located at the G...

The neutral atomic hydrogen (HI) mass function (HIMF) describes the distribution of the HI content of galaxies at any epoch; its evolution provides an important probe of models of galaxy formation and evolution. Here, we report Giant Metrewave Radio Telescope HI 21cm spectroscopy of blue star-forming galaxies at $z\approx0.20-0.42$ in the Extended...

We describe the design, data analysis, and basic results of the Giant Metrewave Radio Telescope Cold-H i AT z ≈ 1 (GMRT-CAT z 1) survey, a 510 hr upgraded GMRT H i 21 cm emission survey of galaxies at z = 0.74−1.45 in the DEEP2 survey fields. The GMRT-CAT z 1 survey is aimed at characterizing H i in galaxies during and just after the epoch of peak...

Voids occupy about 3/4 of the volume of the Universe and contain about 15% of its mass. Due to various observational selection effects, these structure elements and galaxies populating voids, are highly under-explored. This especially relates to the lowest mass galaxies which comprise the main void population. Studying the nearby voids allows us to...

A 32-input, 32 MHz bandwidth, 5-beam, aperture array beamformer has been designed and implemented as part of the Expanded GMRT (eGMRT) proposal. This prototype beamformer development was a precursor to a wideband beamformer (300 MHz bandwidth) which is currently being designed. We present the basic architecture and the optimizations to implement th...

We present a comparison between the average atomic gas mass, 〈 M Atom 〉 (including hydrogen and helium), the average molecular gas mass, 〈 M Mol 〉, and the average stellar mass, 〈 M ⋆ 〉, of a sample of star-forming galaxies at z ≈ 0.75–1.45 to probe the baryonic composition of galaxies in and during the epoch of peak star formation activity in the...

We present a comparison between the average atomic gas mass, $\langle M_{Atom}\rangle$ (including HI and He), the average molecular gas mass, $\langle M_{Mol}\rangle$, and the average stellar mass, $\langle M_*\rangle$, of a sample of star-forming galaxies at $z\approx0.75-1.45$, to probe the baryonic composition of galaxies in and during the epoch...

Green Pea galaxies are low-redshift starburst dwarf galaxies, with properties similar to those of the high-redshift galaxies that reionized the universe. We report the first mapping of the spatial distribution of atomic hydrogen (H i ) in and around a Green Pea, GP J0213+0056 at z = 0.0399, using the Giant Metrewave Radio Telescope (GMRT). Like man...

We describe the design, data analysis, and basic results of the Giant Metrewave Radio Telescope Cold-HI AT z≈1 (GMRT-CAT$z$1) survey, a 510-hour upgraded GMRT HI 21 cm emission survey of galaxies at $z=0.74-1.45$ in the DEEP2 survey fields. The GMRT-CAT$z$1 survey is aimed at characterising HI in galaxies during and just after the epoch of peak sta...

Green Pea galaxies are low-redshift starburst dwarf galaxies, with properties similar to those of the high-redshift galaxies that reionized the Universe. We report the first mapping of the spatial distribution of atomic hydrogen (HI) in and around a Green Pea, GP J0213+0056 at z=0.0399, using the Giant Metrewave Radio Telescope (GMRT). Like many Gr...

Measurements of the atomic hydrogen (HI) properties of high-redshift galaxies are critical to understanding the decline in the star-formation rate (SFR) density of the Universe after its peak ~8-11 Gyr ago. Here, we use ~510 hours of observations with the upgraded Giant Metrewave Radio Telescope to measure the dependence of the average HI mass of s...

Measurements of the atomic hydrogen (HI) properties of high-redshift galaxies are critical to understanding the decline in the star-formation rate (SFR) density of the Universe after its peak ~8-11 Gyr ago. Here, we use ~510 hours of observations with the upgraded Giant Metrewave Radio Telescope to measure the dependence of the average HI mass of s...

Focal Plane Array (FPA) beamformer development is a part of the Expanded Giant Metrewave Radio Telescope (eGMRT) proposal which aims at increasing the Field-of-View (FoV) of the GMRT antennas by a factor of 25. To begin with, a feasibility study to build FPA in the 550-850 MHz band along with a beamformer with 300 MHz bandwidth across 16384 spectra...

We study the relationship between the H i specific angular momentum ($\rm j_{g}$) and the H i mass ($\rm M_{g}$) for a sample of galaxies with well measured H i rotation curves. We find that the relation is well described by an unbroken power law $\rm j_{g}$∝ $\rm M_{g}$α over the entire mass range (107 – 1010.5 M⊙), with α = 0.89 ± 0.05 (scatter 0...

We study the relationship between the H{\sc i} specific angular momentum (j$_{\rm g}$) and the H{\sc i} mass (M$_{\rm g}$) for a sample of galaxies with well measured H{\sc i} rotation curves. We find that the relation is well described by an unbroken power law \jg $\propto$ \mg$^{\alpha}$ over the entire mass range (10$^{7}$-10$^{10.5}$ M$_{\odot}...

Atomic hydrogen is a fundamental constituent of galaxies, and is the primary fuel for star formation. Understanding how the atomic hydrogen content of galaxies evolves with time is thus critical to understanding galaxy evolution. In this article, we discuss how one measures the atomic hydrogen content of distant galaxies, and what these measurement...

We have used the Arecibo Telescope and the Green Bank Telescope to carry out a deep search for H{\sc i}~21\,cm emission from a large sample of Green Pea galaxies, yielding 19 detections, and 21 upper limits on the H{\sc i} mass. We obtain H{\sc i} masses of $\rm M_{HI} \approx (4-300) \times 10^8 \, \rm M_\odot$ for the detections, with a median H{...

The Expanded GMRT (eGMRT) project proposes to increase the field-of-view (FoV) of GMRT antennas using a 144-element Focal Plane Array (FPA) and 30-beam, 300 MHz bandwidth, 16384-spectral channel beamformer. Through architectural optimization and use of FPGA re-configurability, we have implemented a 32-element, 5-beam, 32 MHz bandwidth, 1024-channel...

We have used the Arecibo Telescope and the Green Bank Telescope (GBT) to carry out a deep search for H i 21 cm emission from a large sample of “Green Pea” galaxies, yielding 19 detections, and 21 upper limits on the H i mass. We obtain H i masses of M H i ≈ (4–300) × 10 ⁸ M ⊙ for the detections, with a median H i mass of ≈ 2.6 × 10 ⁹ M ⊙ ; for the...

Baryonic processes in galaxy evolution include the infall of gas onto galaxies to form neutral atomic hydrogen, which is then converted to the molecular state (H2), and, finally, the conversion of H2 to stars. Understanding galaxy evolution thus requires an understanding of the evolution of stars and of neutral atomic and molecular hydrogen. For th...

We present a multifrequency study of eclipse properties of a transitional redback millisecond pulsar J1227−4853 discovered in 2014 with the GMRT. Emission from this pulsar is eclipsed at 607 MHz for about 37% of its orbit around the superior conjunction. We observe eclipse ingress and egress transitions that last for 12% and 15% of its orbit, respe...

We report the discovery of two remarkable high-opacity H i 21 cm absorbers against low-luminosity active galactic nuclei (AGNs), at z = 1.2166 toward J0229+0044 and at z = 1.1630 toward J0229+0053. The absorbers were detected in an unbiased Giant Metrewave Radio Telescope survey for H i 21 cm absorption against radio sources in the DEEP2 survey fie...

We present a multi-frequency study of eclipse properties of a transitional redback millisecond pulsar J1227$-$4853 discovered in 2014 with the GMRT. Emission from this pulsar is eclipsed at 607 MHz for about 37% of its orbit around the superior conjunction. We observe eclipse ingress and egress transition which last 12% and 15% of its orbit respect...

The Expanded GMRT (eGMRT) is a proposal to study three expansions to the Giant Metrewave Radio Telescope (GMRT) - increasing the field-of-view (FoV), the angular resolution and the sensitivity to the extended radio emission. A study of improvement in sensitivity and increased FoV for the eGMRT configuration is described in [1]. Here, we focus on th...

We construct mass models of eight gas rich dwarf galaxies that lie in the Lynx–Cancer void. From NFW fits to the dark matter halo profile, we find that the concentration parameters of haloes of void dwarf galaxies are similar to those of dwarf galaxies in normal density regions. We also measure the slope of the central dark matter density profiles,...

We construct mass models of eight gas rich dwarf galaxies that lie in the Lynx-Cancer void. From NFW fits to the dark matter halo profile, we find that the concentration parameters of halos of void dwarf galaxies are similar to those of dwarf galaxies in normal density regions. We also measure the slope of the central dark matter density profiles,...

We present statistical analysis of a fluence limited sample of over 1100 giant pulses from the Crab pulsar, with fluence > 130 Jy ms at ~1330 MHz. These were detected in ~260 hours of observation with the National Centre for Radio Astrophysics (NCRA)-15m radio telescope. We find that the pulse energy distribution follows a power law with index $\al...

We present statistical analysis of a fluence-limited sample of over 1100 giant pulses from the Crab pulsar, with fluence > 130 Jy ms at ∼1330 MHz. These were detected in ∼260 h of observation with the National Centre for Radio Astrophysics (NCRA) 15 m radio telescope. We find that the pulse-energy distribution follows a power law with index $\rm \a...

This presentation is a combination of the following two talks delivered at META-2019. It describes the beamformer development activity for the Expanded GMRT project. Buch et al. - FPGA-based Focal Plane Array Beamformer Architectures for the Expanded GMRT Dixit et al. - Testing a Multi-element, Multi-beam, FPA Beamformer in Free-space Test Range

We have used the upgraded Giant Metrewave Radio Telescope to carry out a deep (117 on-source hours) L-band observation of the Extended Groth Strip, to measure the average neutral hydrogen (HI) mass and median star formation rate (SFR) of star-forming galaxies, as well as the cosmic HI mass density, at $0.2 < z < 0.4$. This was done by stacking the...

Carbon radio recombination lines (CRRLs) trace the interface between molecular and atomic gas. We present Giant Metrewave Radio Telescope observations of CRRLs, Cα244–Cα250 towards Cassiopeia A. We use a novel technique of stacking the emission lines in the visibility domain to obtain, for the first time, sub-pc resolution optical depth maps of the...

Carbon Radio Recombination Lines trace the interface between molecular and atomic gas. We present GMRT observations of Carbon Radio Recombination Lines (CRRL), C$\alpha$244-C$\alpha$250 towards Cassiopeia A. We use a novel technique of stacking the emission lines in the visibility domain to obtain, for the first time, sub-pc resolution optical dept...

The Expanded GMRT (eGMRT) is a proposal for enhancing the scientific capabilities of the Giant Metrewave Radio Telescope (GMRT). The three expansions proposed as part of the eGMRT are - increasing the field of view, increasing the angular resolution (adding long baselines) and improving the sensitivity (increasing the number of short baselines) to...

With 30 antennas and a maximum baseline length of 25 km, the Giant Metrewave Radio Telescope (GMRT) is the premier low-frequency radio interferometer today. We have carried out a study of possible expansions of the GMRT, via adding new antennas and installing focal plane arrays (FPAs), to improve its point-source sensitivity, surface brightness sen...

With 30 antennas and a maximum baseline length of 25 km, the Giant Metrewave Radio Telescope (GMRT) is the premier low-frequency radio interferometer today. We have carried out a study of possible expansions of the GMRT, via adding new antennas and installing focal plane arrays (FPAs), to improve its point-source sensitivity, surface brightness sen...

Half-dozen of extreme representatives of void dwarf galaxy population were found in our study of evolutionary status of a hundred galaxies in the nearby Lynx-Cancer void. They are very gas-rich, extremely low-metallicity [7.0 < 12+log(O/H)< ~7.3] objects, with blue colours of outer parts. The colours indicate the ages of the oldest visible stellar...

We present a detailed analysis of post-correlation (PC) beamforming (i.e., beamforming which involves only phased sums of the correlation of the voltages of different antennas in an array), and compare it with the traditionally used incoherent and phased beamforming techniques. Using data from the GMRT we show that PC beam formation results in a ma...

Mass and specific angular momentum are two fundamental physical parameters of galaxies.We present measurements of the baryonic mass and specific angular momentum of 11 void dwarf galaxies derived from neutral hydrogen (H I) synthesis data. Rotation curves were measured using 3D and 2D tilted-ring fitting routines, and the derived curves generally o...

We report a deep Giant Metrewave Radio Telescope search for Galactic HI 21-cm absorption towards the quasar B0438-436, yielding the detection ofwide, weakHI 21-cm absorption, with a velocity-integrated HI 21-cm optical depth of 0.0188 ± 0.0036 km s⁻¹. Comparing this with theHI column density measured in the ParkesGalactic All-Sky Survey gives a col...

We present a detailed analysis of post-correlation beamforming (i.e. beamforming which involves only phased sums of the correlation of the voltages of different antennas in an array), and compare it with the traditionally used incoherent and phased beamforming techniques. Using data from the GMRT we show that post-correlation beamformation results...

Half-dozen of extreme representatives of void dwarf galaxy population were found in our study of evolutionary status of a hundred galaxies in the nearby Lynx-Cancer void. They are very gas-rich, extremely low-metallicity [7.0 < 12 + log( O/H ) < ∼ 7.3] objects, with blue colours of outer parts. The colours indicate the ages of the oldest visible st...

We present high resolution Hi 21cm Giant Meterwave Radio Telescope (GMRT) observations of the superthin galaxy FGC1540 with a spatial resolution of 10″ × 8″ and a spectral resolution of 1.73 kms⁻¹ and an rms noise of 0.9 mJy per beam. We obtain its rotation curve as well as deprojected radial Hi surface density profile by fitting a 3-dimensional ti...

We present high resolution H{\sc i} 21cm Giant Meterwave Radio Telescope (GMRT) observations of the superthin galaxy FGC1540 with a spatial resolution of 10$''$ $\times$ 8$''$ and a spectral resolution of 1.73 kms$^{-1}$ and an rms noise of 0.9 mJy per beam. We obtain its rotation curve as well as deprojected radial H{\sc i} surface density profile...

Mass and specific angular momentum are two fundamental physical parameters of galaxies. We present measurements of the baryonic mass and specific angular momentum of 11 void dwarf galaxies derived from neutral hydrogen (H{\sc i}) synthesis data. Rotation curves were measured using 3D and 2D tilted ring fitting routines, and the derived curves gener...

This presentation describes the initial developments and testing in the FPGA-based FPA beamformer for the Expanded GMRT proposal.

The cosmos is strange and beautiful, full of unanticipated objects. The only way to understand it is to observe it–in as many different ways as possible. In this article, we take a look at the wonders revealed upon observing radio emissions from celestial objects through the Giant Metre-wave Radio Telescope (GMRT), near Pune, India.

We have used the Arecibo Telescope to carry out one of the deepest-ever integrations in radio astronomy, targetting the redshifted conjugate satellite OH 18 cm lines at $z \approx 0.247$ towards PKS1413+135. The satellite OH 1720 and 1612 MHz lines are respectively in emission and absorption, with exactly the same line shapes due to population inve...

We use observations made with the Giant Metrewave Radio Telescope (GMRT) to probe the neutral hydrogen (H I) gas content of field galaxies in the VIMOS VLT Deep Survey (VVDS) 14h field at z ≈ 0.32. Because the HI emission from individual galaxies is too faint to detect at this redshift, we use an HI spectral stacking technique using the known optic...

We use observations made with the Giant Metrewave Radio Telescope (GMRT) to probe the neutral hydrogen (HI) gas content of field galaxies in the VIMOS VLT Deep Survey (VVDS) 14h field at $z \approx 0.32$. Because the HI emission from individual galaxies is too faint to detect at this redshift, we use an HI spectral stacking technique using the know...

The Giant Metrewave Radio Telescope (GMRT) is today a frontline international facility for lowfrequency radio astronomy, that has produced several exciting and important new results in the 15 years that it has been operational. To keep the GMRT competitive in the global arena in the future, a major upgrade of the observatory is nearing completion t...

The extended Schmidt law (ESL) is a variant of the Schmidt law which relates the surface densities of gas and star formation, with the surface density of stellar mass added as an extra parameter. We empirically investigate for the first time whether low metallicity faint dwarf irregular galaxies (dIrrs) follow the ESL. Here we consider the `global'...

In this paper, we review the results of interplanetary scintillation (IPS) observations made with the legacy system of the Ooty Radio Telescope (ORT) and compare them with the possibilities opened by the upgraded ORT, the Ooty Wide Field Array (OWFA). The stability and the sensitivity of the legacy system of ORT allowed the regular monitoring of IP...

We describe here an ongoing upgrade to the legacy Ooty Radio Telescope (ORT).The ORT is a cylindrical parabolic cylinder 530mx30m in size operating at a frequency of 326.5 (or z ~ 3.35 for the HI 21cm line). The telescope has been constructed on a north-south hill slope whose gradient is equal to the latitude of the hill, making it effectively equi...

We developed a generic formalism to estimate the event rate and the redshift distribution of Fast Radio Bursts (FRBs) in our previous publication (Bera et al. 2016), considering FRBs are of an extragalactic origin. In this paper we present (a) the predicted pulse widths of FRBs by considering two different scattering models, (b) the minimum total e...

We derive the specific baryonic angular momentum of five gas rich dwarf galaxies from HI kinematics complemented by stellar mass profiles. Since the gas mass of these galaxies is much larger than the stellar mass, the angular momentum can be determined with relatively little uncertainty arising from the uncertainties in the stellar mass to light ra...

We present resolved H I and CO observations of three galaxies from the HIghMass sample, a sample of H I assive (MH1> 10 ¹⁰M⊙ ), gas-rich (MH 1 in the top 5% for their M) galaxies identified in the ALFALFA survey. Despite their high gas fractions, these are not low-surface-brightness galaxies and have typical specific star formation rates (SFR M) fo...

We present resolved and CO observations of three galaxies from the HIghMass sample, a sample of -massive (M_(H I) > 10^(10) M⊙), gas-rich (M_(H I) in the top 5% for their M *) galaxies identified in the ALFALFA survey. Despite their high gas fractions, these are not low-surface-brightness galaxies and have typical specific star formation rates (SFR...

We present HI 21cm and optical observations of UGC 3672 which is located near the centre of the nearby Lynx-Cancer void. We find that UGC 3672 consists of an approximately linearly aligned triplet of gas rich dwarfs with large scale velocity continuity along the triplet axis. The faintest component of the triplet is extremely gas-rich (MHI/LB ~ 17)...

We present resolved HI and CO observations of three galaxies from the HIghMass sample, a sample of HI-massive ($M_{HI} > 10^{10} M_\odot$), gas-rich ($M_{HI}$ in top $5\%$ for their $M_*$) galaxies identified in the ALFALFA survey. Despite their high gas fractions, these are not low surface brightness galaxies, and have typical specific star format...

We present the observations and first results from the FIGGS2 survey. FIGGS2 is an extension of the earlier Faint Irregular Galaxies GMRT survey (FIGGS) towards faint luminosity end. The sample consists of 20 galaxies of which 15 were detected in HI 21cm line using the Giant Meter-wave Radio Telescope (GMRT). The median blue band magnitude of our s...

We present the results of H i spectral stacking analysis of Giant Metrewave Radio Telescope (GMRT) observations targeting the COSMOS field. The GMRT data cube contains 474 field galaxies with redshifts known from the zCOSMOS-bright 10k catalogue. Spectra for the galaxies are co-added and the stacked spectrum allows us to make a ∼3σ measurement of t...

We revisit the Hi size-mass (DHI-MHI) relation of galaxies with a sample of more than 500 nearby galaxies covering over five orders of magnitude in Hi mass and more than ten B-band magnitudes. The relation is remarkably tight with a scatter σ ∼0.06 dex, or 14%. The scatter does not change as a function of galaxy luminosity, Hi richness or morpholog...

Assuming that Fast Radio Bursts (FRBs) are of extragalactic origin, we have developed a formalism to predict the FRB detection rate and the redshift distribution of the detected events for a telescope with given parameters. We have adopted FRB 110220, for which the emitted pulse energy is estimated to be E0 = 5.4 × 1033 J, as the reference event. T...

We present the results of a study of the amount and distribution of cold atomic gas, as well its correlation with recent star formation in a sample of extremely faint dwarf irregular galaxies. Our sample is drawn from the Faint Irregular Galaxy GMRT Survey (FIGGS) and its extension, FIGGS2. We use two different methods to identify cold atomic gas....

Observations are presented of the isolated dwarf irregular galaxy And IV made with the Hubble Space Telescope Advanced Camera for Surveys and the Giant Metrewave Radio Telescope in the 21 cm HI line. We determine the galaxy distance of $7.17\pm0.31$ Mpc using the Tip of Red Giant Branch method. The galaxy has a total blue absolute magnitude of -12....

We present H i 21 cm emission observations of the z ∼ 0.006 32 subdamped Lyman-α absorber (sub-DLA) towards PG 1216+069 made using the Arecibo Telescope and the Very Large Array (VLA). The Arecibo H i 21cm spectrum corresponds to an H i mass of ∼3.2 × 107 M⊙, two orders of magnitude smaller than that of a typical spiral galaxy. This is surprising s...

We are conducting a survey for pulsars and transients using the Giant Metrewave Radio Telescope (GMRT). The GMRT High Resolution Southern Sky (GHRSS) survey is an off-Galactic-plane (|b|>5) survey in the declination range -40 deg to -54 deg at 322 MHz. With the high time (up to 30.72 micro-sec) and frequency (up to 0.016275 MHz) resolution observin...

We present HI 21cm emission observations of the z ~ 0.00632 sub-damped Lyman-alpha absorber (sub-DLA) towards PG1216+069 made using the Arecibo Telescope and the Very Large Array (VLA). The Arecibo 21cm spectrum corresponds to an HI mass of ~ 3.2x10^7 solar masses, two orders of magnitude smaller than that of a typical spiral galaxy. This is surpri...

We investigate the existence of a Kennicutt-Schmidt type relation between ΣSFR and Σgas, atomic in the HI dominated regions of nearby spirals and dwarf irregulars.

We study the Kennicutt-Schmidt relation between average star formation rate and average cold gas surface density in the Hi dominated ISM of nearby spiral and dwarf irregular galaxies. We divide the galaxies into grid cells varying from sub-kpc to tens of kpc in size. Grid-cell measurements of low SFRs using H-alpha emission can be biased and scatte...

We present a detailed study of the extremely isolated Sdm galaxy UGC 4722 (MB = −17.4) located in the nearby Lynx-Cancer void. UGC 4722 is a member of the Catalogue of Isolated Galaxies, and has also been identified as one of the most isolated galaxies in the Local Supercluster. Optical images of the galaxy however show that it has a peculiar morph...

XSS J12270-4859 is an X-ray binary associated with the Fermi LAT gamma-ray source 1FGL J1227.9$-$4852. In 2012 December, this source underwent a transition where the X-ray and optical luminosity dropped and the spectral signatures of an accretion disc disappeared. We report the discovery of a 1.69 millisecond pulsar (MSP), PSR J1227-4853, at a disp...

We model the observed vertical distribution of the neutral hydrogen (H i) in the faint (MB ∼ −13.7 mag) edge-on dwarf irregular galaxy KK250. Our model assumes that the galaxy consists of axisymmetric, coplanar gas and stellar discs in the external force-field of a spherical dark matter halo, and in vertical hydrostatic equilibrium. The velocity di...

We present first results of the study of a set of exceptional H I sources identified in the 40% ALFALFA extragalactic H I survey catalog alpha.40 as both being H I massive (M-H I > 10(10) M-circle dot) and having high gas fractions for their stellar masses: the HIghMass galaxy sample. We analyze UV- and optical-broadband and H alpha images to under...

We present resolved HI observations of two galaxies, UGC 9037 and UGC 12506, members of a rare subset of galaxies detected by the ALFALFA extragalactic HI survey characterized by high HI mass and high gas fraction for their stellar masses. Both of these galaxies have M$_*>10^{10}$ M$_\odot$ and M$_\text{HI}>$ M$_*$, as well as typical star formatio...

We use data for faint (M_B > -14.5) dwarf irregular galaxies drawn from the FIGGS survey to study the correlation between the atomic gas density (Sigma_gas,atomic) and star formation rate (Sigma_SFR) in the galaxies. The estimated gas phase metallicity of our sample galaxies is Z ~ 0.1 Z_sun. Understanding star formation in such molecule poor gas i...

We present first results of the study of a set of exceptional HI sources identified in the 40% ALFALFA extragalactic HI survey catalog alpha.40 as being both HI massive (M_HI > 10^10 Msun) and having high gas fractions for their stellar masses: the HIghMass galaxy sample. We analyze UV- and optical-broadband and Halpha images to understand the natu...

We use MERLIN, VLA and VLBA observations of Galactic H i absorption towards 3C 138 to estimate the structure function of the H i opacity fluctuations at au scales. Using Monte Carlo simulations, we show that there is likely to be a significant bias in the estimated structure function at signal-to-noise ratios characteristic of our observations, if...

We present new H I spectral line imaging of the extremely metal-poor, star-forming dwarf irregular galaxy Leo P. Our H I images probe the global neutral gas properties and the local conditions of the interstellar medium (ISM). The H I morphology is slightly elongated along the optical major axis. We do not find obvious signatures of interaction or...

Detailed knowledge of the foreground structure on the angular scales of ~ 1° to sub-arcminute will be essential for extracting the redshifted 21-cm HI signal from the observed data. We have presented results from the GMRT observations at 153 MHz, which was used to characterize the statistical properties of the diffuse radiation, using the multi-fre...

We report results from a programme aimed at investigating the temperature of neutral gas in high-redshift damped Lyman-$\alpha$ absorbers (DLAs). This involved (1) HI 21cm absorption studies of a large DLA sample, (2) VLBI studies to measure the low-frequency quasar core fractions, and (3) optical/ultraviolet spectroscopy to determine DLA metallici...

For radio interferometric arrays with a sufficient number of redundant spacings the multiplicity of measurements of the same sky visibility can be used to determine both the antenna gains as well as the true visibilities. Many of the earlier approaches to this problem focused on linearized versions of the relation between the measured and the true...

We have carried out deep, high velocity resolution, interferometric Galactic H i-21 cm absorption spectroscopy towards 32 compact extragalactic radio sources with the Giant Metrewave Radio Telescope (GMRT) and the Westerbork Synthesis Radio Telescope (WSRT). The optical depth spectra for most sources have root-mean-square noise values ≲10−3 per 1 k...

We discuss physical conditions in Galactic neutral hydrogen based on deep, high-velocity resolution interferometric H i-21 cm absorption spectroscopy towards 33 compact extragalactic radio sources. The H i-21 cm optical depth spectra have root-mean-square noise values ≲10−3 per 1 km s−1 velocity channel, i.e. sufficiently sensitive to detect H i-21...

We use the measured B-band axial ratios of galaxies from an updated catalogue of Local Volume galaxies to determine the intrinsic shape of dwarf irregular galaxies (de Vaucouleurs’ morphological types 8–10). We find that the shapes change systematically with luminosity, with fainter galaxies being thicker. In particular, we divide our sample into s...

Using the Tremaine-Weinberg method, we measure the speed of the HI bar seen in the disk of NGC 3741. NGC 3741 is an extremely gas rich galaxy with an {H\,{\sc i}} disk which extends to about 8.3 times its Holmberg radius. It is also highly dark matter-dominated. Our calculated value of the pattern speed $\Omega_p$ is 17.1 $\pm$ 3.4 km $\textrm{s}^{...

We measure the neutral atomic hydrogen (H i) gas content of field galaxies at intermediate redshifts of z ∼ 0.1 and ∼0.2 using hydrogen 21-cm emission lines observed with the Westerbork Synthesis Radio Telescope. In order to make high signal-to-noise ratio detections, an H i signal stacking technique is applied: H i emission spectra from multiple g...

We present a detailed study of an estimator of the H i column density, based on a combination of H i 21 cm absorption and H i 21cm emission spectroscopy. This ‘isothermal’ estimate is given by NHI, ISO = 1.823 × 1018 ∫ [τtot × TB / [ 1 − e−τtot]dV, where τtot is the total H i 21cm optical depth along the sightline and TB is the measured brightness...

As part of the ongoing upgrade of the GMRT observatory, the monitor and control (M&C) system is being upgraded to a modern specification driven system. The basic building block of the proposed M&C framework is a SACE node which provides command, response and event data streaming interfaces to the child and parent nodes running locally or remotely i...